Numerous methods are known for the production of carboxylic amides by reaction of the corresponding carboxylic acid, anhydride or lower alkyl esters thereof with ammonia. While these reactions can be accomplished in the absence of catalyst at elevated temperature and very high pressures (see U.S. Pat. No. 3,253,006) the trend has been to the use of processes which do not require the use of high pressure and which still provide high conversion with high selectivity to the desired amide product. Accordingly, catalytic procedures have been developed whereby the reaction of the carboxylic moiety with ammonia can be achieved by substantially reduced pressures and, in some cases, even at atmospheric pressure.
One such atmospheric amidation process is described in U.S. Pat. No. 2,013,108 and involves passing gaseous ammonia into the fatty acid melt which contains a surface catalyst. Useful surface catalysts for the process are solid inorganic substances such as bleaching earths, fuller's earth, silica gel, natural or synthetic zeolites, oxides and phosphates of aluminum, thorium, tungsten, cerium, praseodymium, neodymium, and lanthanum, bauxite, charcoal, activated carbon, pumice, and the like. With this process it typically requires forty-eight or more hours to achieve acceptable levels of conversion and, as a result, some of the amide is dehydrated to the corresponding nitrile.
Since the yield of the desired amide product can be significantly reduced by this competing reaction, i.e. dehydration of the amide to nitrile, long reaction times generally cannot be tolerated. Accordingly, processes requiring shorter reaction times and employing reaction conditions which minimize undesirable by-product formation have been developed. One such process for the production of carboxylic acid amides is disclosed in U.S. Pat. No. 3,816,483. The process can be conducted at atmospheric pressure and, as a result of the shorter reaction times required, the formation of undesirable nitrile by-product is reduced. Reaction of the carboxylic acid and ammonia is acccomplished in the presence of a reaction-soluble catalyst of a Group IVb or Vb metal, preferably a compound of titanium, zirconium or tantalum.